Microelectromechanical system assembly and testing device

ABSTRACT

A novel MEMS assembly and testing system that utilizes a scanning electron microscope (SEM) having 5 axes of freedom as the imaging instrument. Microgrippers or other tools mounted at the end of a linear motion feed through device having a motion resolution of about 10 nanometers are used as the manipulator. All of the assembly features are located inside of a vacuum chamber to permit operation of the SEM imaging system. A variety of other auxiliary devices that support the MEMS assembly and testing system are also included to enhance the capabilities thereof.

FIELD OF THE INVENTION

The present invention relates to a device for the assembly ofmicroelectromechanical systems (MEMS).

BACKGROUND OF THE INVENTION

Assembly of MEMS is recognized as difficult for three principal reasons.First, the optical imaging methods commonly used to image small MEMSparts during assembly lack sufficient resolution and depth of field tomake accurate imaging of the very small parts practical. Secondly, theparts undergoing assembly are usually manipulated with amicromanipulator equipped with a small set of grippers. The conventionalmicromanipulator has barely the control necessary for assembly of smallmechanical and electromechanical parts. Thirdly, most light microscopesdo not have stages of sufficient axes of freedom to manipulate thedevice so that it is placed properly for precise placement of parts bythe grippers located on the micromanipulator.

U.S. Pat. No. 5,559,329 issued Sep. 24, 1996 describes an apparatus formeasuring the interfacial properties of fiber-matrix composites. Theapparatus includes a linear motion feedthrough for pushing an indentoron a fiber end, a load cell for sensing indentor load and a scanningelectron microscope (SEM) for magnifying the material in order to alignthe indentor with a fiber end undergoing testing. The SEM includes avacuum chamber for housing the indentor, the load cell and a hot stagemodule. Data acquisition and recording devices as well as an imagingcomputer for recording images of the materials during testing are alsodescribed.

OBJECT OF THE INVENTION

It is an object of the present invention to relate the capabilities ofthe devices described in the foregoing U.S. Pat. No. 5,559,329 to thefield of MEMS assembly through the presentation of a novel MEMS assemblydevice that utilizes the general approach of this patent.

SUMMARY OF THE INVENTION

According to the present invention there is provided a novel MEMSassembly and testing system that utilizes a scanning electron microscope(SEM) having 5 axes of freedom as the imaging instrument. This imaginginstrument has a resolution and depth of focus about 100 times greaterthan currently used optically based imaging systems. Microgrippers orother tools mounted at the end of a linear motion feed through devicehaving a motion resolution of about 10 nanometers are used as themanipulator. All of the assembly features are located inside of a vacuumchamber to permit operation of the SEM imaging system. A variety ofother auxiliary devices that support the MEMS assembly and testingsystem are also included to enhance the capabilities thereof.

DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic diagram of the MEMS assembly device of the presentinvention.

FIG. 2 is perspective representation of the assembly sage of the MEMSassembly device of the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, the MEMS assembly device 10 of the presentinvention comprises: a vacuum chamber 12, an SEM 14, a linear motionfeedthrough device 16, connected via a shaft 18 to a microgripper orother suitable tool as described hereinafter 20 and including analignment guide 17, an assembly stage 22, a hot stage heater 24, an SEMstage 26 and a stage platform 28. The various other elements of MEMSassembly device 10 that support the forgoing essential elements include:stage controls 30 that provide control of stage platform 28 movement inthe x, y, and z directions as well as permit tilting and rotation ofstage platform 28 and associated assembly stage 22 and SEM stage 26,linear motion feedthrough controller 32, acoustic emission sensor 34,load cell 36, piezoelectric positioner 35, vacuum gauge 38, X-raymicroanalysis system 40 and SEM column 42. The roles of these variouselements and devices will be explained more fully below.

A suitable linear motion feedthrough 16 is manufactured by HuntingtonMechanical Laboratories as model MFL-275-4. Linear motion feedthrough 16is connected to feedthrough controller 32 by cable 44. A suitablecontroller 32 is manufactured by Huntington Laboratories and includes amodel MLC-1 indexer and a model ssp-500 hand held programmer. Such afeedthrough device provides a 20 pound load force capacity and a minimumdrive velocity of about 0.127 μm/s or about 7.62 μm/min. This load forcecapacity and relatively slow drive speed are suitable for MEMS assemblyoperations. Linear motion feedthrough device 16 can also be equipped toactivate a buzzer or other audible alarm whenever microgripper 20contacts a conductive surface. Such a modification can be helpful whenapproaching a surface or part very slowly. It typically provides anearlier warning of contact than the load cell described hereinafter andis somewhat more reliable than the acoustic emission sensor describedbelow for determining when a surface has been contacted. The use of avariety of redundant systems of this type to provide feedback regardingvarious situations encountered during an assembly operation is highlydesirable and preferred.

SEM 14 includes vacuum chamber 12. This provides an excellent inertenvironment that assures that no contamination due to dust or othercontaminants occurs. Vacuum chamber 12 is preferably mounted on avibration isolation table 46 that dampens out external vibrations foreven greater accuracy in the assembly operation.

Vacuum chamber 12 also houses a linear motion feedthrough shaft 18microgrippers 20, load cell 36, acoustic emission sensor 34 and apiezoelectric positioner 35, as well as the various stages, assembly 22,hot stage 24, and SEM 26 and platform 28. Alignment guide 17 alignsfeedthrough shaft 18 through a passageway in vacuum chamber 12.

A suitable SEM 14 is one previously manufactured by Cambridge asStereoscan 90, Model B includes a backscatter electron detector 50, asecondary electron detector 52 as well as X-ray microanalysis system 40.X-ray microanalysis system 40 includes a digital imaging computer 72that is interconnected with secondary electron detector 52 and X-raymicroanalysis system 40 via cables 74 and running suitable imageanalysis and display software. A suitable digital imaging computer 72can be any of the well known such devices such as a 486 or better PC.Computer 72 is used to acquire, record, transmit and view the assemblyoperation. Imaging in an SEM is also advantageous in that exposedconductors when under power image significantly brighter under the SEMthereby allowing an operator to readily check circuitry duringoperation.

Acoustic emission sensor 34 mounted on feedthrough shaft 18 is one form,in addition to optical imaging, for sensing contact between two partsbeing assembled within vacuum chamber 12 with microgrippers 20. Thesound of contact between two parts is readily detected by such a deviceand provides an additional sensory perception of activity occurringwithin vacuum chamber 12 during assembly. A suitable acoustic emissionsensor is manufactured by Physical Acoustics Corporation as model μ30.

Locating and positioning of parts in the assembly is preferablyperformed in two primary ways. The “shop floor” or “assembly area” canbe precisely moved using the X, Y, and Z tilt and rotate movements ofSEM 14. Alternatively, using piezoelectric positioner 35 connected tolinear motion feedthrough device 16 the X, Y, Z movements ofmicrogrippers 20 (or another suitable tool) can be performed.

Suitable piezoelectric positioning devices 35, sometimes referred to as“nanopositioners”, are commercially available from Polytec PI, Inc., 23Midstate Drive, Suite 212, Auburn, Mass. 01501 as models P-280, P-281and P-282 XY and XYZ PZT Flexure NanoPositioners.

Similarly, load cell 36 connected to the linear axis of feedthroughshaft 18 within vacuum chamber 12 permits careful control of various“pushing” or “pulling” operations occurring during an assembly operationby providing feed back to the operator as individual parts as insertedinto one another or other pressure sensitive operations are performed. Asuitable load cell is manufactured by Sensotec as Model 31 that can befitted with a variety of sensing heads ranging in capacity from aboutone or two pounds up to about 25 pounds.

Both load cell 36 and acoustic emission sensor 34 are monitored by asuitable detection device 68 such as a computer connected to thesedevices via cables 70.

Vacuum gauge 38 provides the means for monitoring the pressure withinvacuum chamber 12. Such devices are well known in the art and commonlyassociated with vacuum chambers of the type utilized in conjunction withSEMs.

Referring now to FIG. 2 that depicts schematically the MEMS assemblystage 22 of the apparatus of the present invention, MEMS assembly stage22 includes a hot stage heater 24 such as that manufactured OxfordInstruments Limited as model H1005 controlled by a suitable digitaltemperature controller (not shown). The hot stage heater 24 permitscareful and accurate control of temperature on the “shop floor” orassembly area 58, i.e. the area atop SEM sample holder or stage 28.Additionally, the presence of hot stage 24 permits heating of partsbeing assembled to cure, for example, thermoset polymers or to braze orsolder parts. Elevated temperature testing of parts, materials orassemblies or “burn in” temperature operating conditions of electronicor mechanical assemblies can also be accomplished with thisconfiguration. The temperature of hot stage 24 is monitored andcontrolled by hot stage controller 54 connected to hot stage 24 viacable 56.

Mounted about the periphery of SEM stage 26 are one or more holders, twoin number in the accompanying drawings, identified by the numerals 60and 62. Each of holders 60 and 62 includes apertures or othercontainment devices such as apertures 64 for placement of partsnecessary in the microassembly operation. Another use of apertures 64,as shown in the case of holder 62 in accompanying FIG. 2 is for theplacement of, for example, a container 66 of a vacuum stable adhesiveand an applicator 68 therefor. The placement of parts, adhesives etc. inthese locations allows ready accessibility thereto during assemblyoperations. Unless it is desired that “tools”, parts or other elementsplaced in apertures 64 be heated prior to use, it is highly desirablethat an air gap 70 or some suitable insulation layer be provided betweenassembly stage 58 and holders 60-62.

The MEMS assembly and testing device described herein can be equippedwith a wide array of additional imaging methods including: secondaryelectron imaging, that provides imaging of the structure beingassembled; backscatter imaging that can be set to vary contrast withrespect to atomic number; specimen current imaging that identifieselectrically conductive pathways; electron fluorescent imaging thatimages based upon the sample's ability to give off light while beingexcited by an electron beam as well as X-ray microanalysis that canprovide elemental analysis of virtually any location being imaged. Alight microscope can also be incorporated.

As will be apparent to the skilled artisan, a wide variety of “tools”can be substituted for the microgrippers referred to hereinabove andcommonly used in MEMS assembly operations. For example, voltage probes,punches, hooks, cutting instruments etc. can all be used in place of themicrogripper 20 and such additional “tools” can be stored in, forexample, apertures 64 in holders 62 of the accompanying drawings orother similar retaining means.

In use, the appropriate “tools”, parts and other materials are locatedin holders 60 and 62 or upon assembly area 58 and a microgripper orother suitable assembly or testing device attached to feedthrough shaft18. The vacuum chamber is then sealed and evacuated in accordance withnormal procedures. Once scanning is commenced, the initial MEMS part(generally attached to the assembly stage surface 58) is brought to theappropriate location and orientation through manipulation of stagecontrols 30. Linear motion feedthrough device 16 is slowly brought intoview. Careful movement of both the stage controls 30 and the linearmotion feedthrough device 16 allows for the lifting of variouscomponents from holders 60 and 62 and permits them to be placedprecisely in their appropriate assembly locations.

Attachment of the various parts being assembled can be achieved in avariety of ways, for example, by friction fit, through the use of vacuumstable thermoset or thermoplastic polymers, heat, electron beam or UVcured, or by soldering or brazing.

During assembly, the force used to press or hold the parts in place canbe measured by load cell 36 and acoustic emission sensor 34 can be usedto provide an audible confirmation of contact between parts, as can thepreviously described conductive path technique.

As will be apparent to the skilled artisan, more than one linear motionfeedthrough device 16 could be installed in a single MEMS assemblydevice according to the present invention.

Among the myriad of sensors and machines that MEMS assembly device 10could be used to assemble are included, but not limited to:accelerometers such as those used in airbags, automobile control,pacemakers, games, automotive brakes, image stabilizers and otherinertial measurement systems, so-called laboratory on a chip typesensors; flow sensors, optical switches, projection and handhelddisplays; pressure sensors such as those developed for tire pressure andother automotive and industrial applications; miniature read/writeheads; cell phone parts; MEMS devices for radar applications andsteerable antennas; microrelays; and a variety of sensors to measuresuch physical properties as humidity, temperature, vibration etc.

While a significant advantage of MEMS assembly device 10 of the presentinvention is that assembly of prototype and production micro-devices canbe made manually guided by the high resolution and depth of focus of theSEM, the device is also capable of being partly or entirely automatedfor high volume production and testing.

Through the use of appropriate tools, MEMS device 10 is capable of beingused to test virtually any material or device that can be attached toassembly stage 26 in tensile, creep, or compression loading. Among thetests possible is the measurement of properties of virtually anycomponent manufactured using the MEMS process where the smallestdimension needed to be resolved is greater than about 0.05μ. This wouldinclude the testing of gears, posts, and other structural members. MEMSdevice 10 can also be used to measure the mechanical properties ofcoatings or films with such tests as scratch tests, impact tests andhardness tests. Such testing would not only disclose the informationabout the strength of such materials, but it would also providemicromechanical information leading to a micromechanical understandingof the mode of failure which is sometimes the most valuable piece ofinformation. For example, placing a microhardness indentor in place ofthe microgrippers described above allows for the measurement ofmicrohardness while observing the sample at the SEM level. Additionally,the fine control possible with MEMS device 10 will allow the measurementof nanohardness. Furthermore, all such tests can be done at elevatedtemperature and all while imaging the test while it is in progress todetect minute changes such as detecting if failure occurs through theformation of a crack at a load point rather than through deformation.Other tests that can be performed include a variety of fiber tests suchas 3-point and 4-point bend tests, crush tests, and tensile tests.Abrasion testing can also be performed by continually increasing theload placed by an indentor while simultaneously moving the samplebeneath the indentor all while the micromechanics of the test are beingobserved.

As the invention has been described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchmodifications are intended to be included within the scope of theappended claims.

What is claimed is:
 1. A MEMS assembly and testing device comprising: A)a scanning electron microscope; B) a vacuum chamber having walls andwithin which said scanning electron microscope operates; C) a linearmotion feedthrough device having a shaft that passes through a wall ofsaid vacuum chamber and includes a linear motion feedthrough deviceshaft having a terminal end; D) attached to the terminal end and withinsaid vacuum chamber a tool; E) a piezoelectric positioner locatedintermediate said linear motion feedthrough device shaft and said tool;F) in the vicinity of said tool in said vacuum chamber and controlledvia stage controls external to said vacuum chamber a manipulable stageplatform; G) a scanning electron microscope stage mounted on said stageplatform; H) an assembly stage mounted on said scanning electronmicroscope stage; I) about the periphery of said assembly stage at leastone mechanism for holding parts and other materials necessary for theaccomplishment of assembly or testing; and J) an X-ray microanalysissystem for the collection of Xrays and analysis and imaging thereof. 2.The MEMS assembly and testing device of claim 1 wherein said tool is amicrogripper.
 3. The MEMS assembly and testing device of claim 1 furtherincluding intermediate said scanning electron microscope stage and saidassembly stage a hot stage for controllably heating said assembly stageand any parts or materials located thereon.
 4. The MEMS assembly andtesting device of claim 3 wherein said mechanism for holding parts andother materials necessary for the accomplishment of assembly or testingis insulated from said assembly stage.
 5. The MEMS assembly and testingdevice of claim 4 wherein said hot stage and said mechanism for holdingparts and other materials necessary for the accomplishment of assemblyor testing is insulated from said assembly stage by an air gap.
 6. TheMEMS assembly and testing device of claim 1 further includingintermediate said tool and said terminal end, a load cell for themeasurement of load applied to said tool during assembly or testing. 7.The MEMS assembly and testing device of claim 1 further including anacoustic emission sensor for detecting contact between parts beingassembled or tested.
 8. The MEMS assembly and testing device of claim 1further including an alignment guide for said linear motion feedthroughdevice.
 9. A MEMS assembly and testing device comprising: A) a scanningelectron microscope; B) a vacuum chamber having walls and within whichsaid scanning electron microscope operates; C) a linear motionfeedthrough device having a shaft that passes through a wall of saidvacuum chamber and includes a linear motion feedthrough device shafthaving a terminal end; D) attached to the terminal end and within saidvacuum chamber a tool; E) in the vicinity of said tool in said vacuumchamber and controlled via stage controls external to said vacuumchamber a manipulable stage platform; F) a scanning electron microscopestage mounted on said stage platform; G) an assembly stage mounted onsaid scanning electron microscope stage; H) an X-ray microanalysissystem for the collection of X-rays and analysis and imaging thereof.